Apparatus and methods for aberration correction in electron beam based system

ABSTRACT

One embodiment relates to an apparatus for aberration correction in an electron beam lithography system. An inner electrode surrounds a pattern generating device, and there is at least one outer electrode around the inner electrode. Each of the inner and outer electrodes has a planar surface in a plane of the pattern generating device. Circuitry is configured to apply an inner voltage level to the inner electrode and at least one outer voltage level to the at least one outer electrode. The voltage levels may be set to correct a curvature of field in the electron beam lithography system. Another embodiment relates to an apparatus for aberration correction used in an electron based system, such as an electron beam inspection, or review, or metrology system. Other embodiments, aspects and features are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 13/287,682, filed Nov. 2, 2011, the disclosure of which ishereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Agreement No.HR0011-07-9-0007 awarded by DARPA. The Government has certain rights inthe invention.

BACKGROUND

Technical Field

The present invention relates generally to semiconductor manufacturingand related technologies. More particularly, the present inventionrelates to electron beam lithography systems and other electron beambased systems.

Description of the Background Art

As is well-understood in the art, a lithographic process includes thepatterned exposure of a resist so that portions of the resist can beselectively removed to expose underlying areas for selective processingsuch as by etching, material deposition, implantation and the like.Traditional lithographic processes utilize electromagnetic energy in theform of ultraviolet light for selective exposure of the resist. As analternative to electromagnetic energy (including x-rays), chargedparticle beams have been used for high resolution lithographic resistexposure. In particular, electron beams have been used since the lowmass of electrons allows relatively accurate control of an electron beamat relatively low power and relatively high speed.

In general, electron beam lithographic systems may be designed tooperate in either a reflection mode or a transmission mode. In areflection mode, the electron beam is patterned by reflecting the beamfrom a selectively reflective array. If the pattern on the reflectivearray is dynamically changeable, then the array may be referred to as adynamic pattern generator (DPG). In a transmission mode, the electronbeam is patterned by transmitting the beam through a blanker array.

The electron-optical elements of electron beam lithographic systemsgenerally cause imaging aberrations which need to be corrected.Aberration correction is typically performed using multi-pole elements.However, the multi-pole elements used for aberration correction aretypically large and expensive and are limited as to which aberrationscan be corrected.

It is highly desirable to improve lithography systems. The presentdisclosure provides advantageous apparatus and methods for correctingaberrations in an electron beam lithography system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a reflection-mode electron beamlithography system in accordance with an embodiment of the invention.

FIG. 2 is a cross-sectional diagram of a multiple-electrode reflectorstructure for a dynamic pattern generator in accordance with anembodiment of the invention.

FIG. 3A is a planar view of an aberration correction apparatus for areflection-mode system in accordance with an embodiment of theinvention.

FIG. 3B is a cross-sectional view of an aberration correction apparatusfor a reflection-mode system in accordance with an embodiment of theinvention.

FIG. 4 is a planar view of an aberration correction apparatus for areflection-mode system in accordance with another embodiment of theinvention.

FIG. 5 is a simplified block diagram of a transmission-mode electronbeam lithography system in accordance with an embodiment of theinvention.

FIG. 6A is a planar view of an aberration correction apparatus for atransmission-mode system in accordance with an embodiment of theinvention.

FIG. 6B is a cross-sectional view of an aberration correction apparatusfor a transmission-mode system in accordance with an embodiment of theinvention.

FIG. 7 is a planar view of an aberration correction apparatus for atransmission-mode system in accordance with another embodiment of theinvention.

FIG. 8 is a planar view of an apparatus used to correct aberration in anelectron beam based system in accordance with an embodiment of theinvention.

SUMMARY

One embodiment relates to an apparatus for aberration correction in anelectron beam lithography system. An inner electrode surrounds a patterngenerating device, and there is at least one outer electrode around theinner electrode. Each of the inner and outer electrodes has a planarsurface in a plane of the pattern generating device. Circuitry isconfigured to apply an inner voltage level to the inner electrode and atleast one outer voltage level to the at least one outer electrode. Thevoltage levels may be set to correct a curvature of field in theelectron beam lithography system.

Another embodiment relates to an electron beam lithography system. Thesystem includes at least an electron source, illuminationelectron-optics, a pattern generating device, projectionelectron-optics, and a target substrate. An inner electrode isconfigured to surround the pattern generating device, and at least oneouter electrode is configured around the inner electrode. Each of theinner and outer electrodes has a planar surface in a plane of thepattern generating device. Circuitry is configured to apply an innervoltage level to the inner electrode and at least one outer voltagelevel to the at least one outer electrode.

Another embodiment relates to a method of correcting aberration in anelectron beam lithography system. Adjustment is made to an inner voltagelevel applied to an inner electrode surrounding a pattern generatingdevice, where the inner electrode has a planar surface in a plane of thepattern generating device. Adjustment is also made to at least one outervoltage level which is applied to at least one outer electrode aroundthe inner electrode, where the at least one outer electrode also has aplanar surface in the plane of the pattern generating device.

Another embodiment relates to an apparatus for aberration correction inan electron beam based system. An inner electrode surrounds an openingor reflector, and there is at least one outer electrode around the innerelectrode. Each of the inner and outer electrodes has a planar surfacein a plane of the opening or reflector. Circuitry is configured to applyan inner voltage level to the inner electrode and at least one outervoltage level to the at least one outer electrode. The voltage levelsmay be set to correct a curvature of field in the electron beam basedsystem.

Another embodiment relates to a method of correcting aberration in anelectron beam based system. Adjustment is made to an inner voltage levelapplied to an inner electrode surrounding an opening or reflector, wherethe inner electrode has a planar surface in a plane of the opening orreflector. Adjustment is also made to at least one outer voltage levelwhich is applied to at least one outer electrode around the innerelectrode, where the at least one outer electrode also has a planarsurface in the plane of the opening or reflector.

Other embodiments, aspects and features are also disclosed.

DETAILED DESCRIPTION

The present disclosure provides apparatus and methods for theadvantageous correction of aberrations in an electron beam lithographysystem. In accordance with embodiments of the invention, curvature offield and higher-order aberrations may be corrected without adding tothe electron-optical path length. This allows for lower coulombinteraction in the projection arm of the system.

In accordance with one embodiment of the invention, the apparatus andmethods may be advantageously applied to an electron beam projectionsystem operating in a reflection mode. In accordance with anotherembodiment of the invention, the apparatus and methods may beadvantageously applied to a an electron beam projection system operatingin a transmission mode.

Reflection-Mode System

An example of an electron beam lithography system designed to operate ina reflection mode is depicted in FIG. 1. While FIG. 1 shows one exampleof a reflection-mode electron beam system, it is contemplated that theapparatus and methods of the present invention are applicable in a widevariety of reflection-mode electron beam systems.

As depicted in FIG. 1, the example reflection-mode electron beamlithography system 100 includes an electron source 102, illuminationelectron-optics 104, a beam separator 106, an objective electron lens110, a dynamic pattern generator (DPG) 112, projection electron-optics114, and a stage 116 for holding a semiconductor wafer or other targetsubstrate 118 to be lithographically patterned.

The illumination electron-optics 104 is configured to focus andcollimate the electron beam from the electron source 102. Theillumination electron-optics 104 may comprise an arrangement of magneticand/or electrostatic lenses and allows the setting of the currentilluminating the DPG 112.

The beam separator 106 may be configured to receive the incidentelectron beam 105 from the illumination electron-optics 104. In oneimplementation, the beam separator 106 comprises a magnetic prism. Whenthe incident beam 105 travels through the magnetic fields of the prism,its trajectory is bent towards the objective electron-optics 110. Theobjective electron-optics 110 receives the incident beam from theseparator 106 and decelerates and focuses the incident electrons as theyapproach the DPG 112.

The DPG 112 may include a two-dimensional array of pixels. As oneexample, the dimensions of the array may be 4096×248 pixels. Variousother dimensions of the array may also be implemented. As describedfurther below in relation to FIG. 2, each pixel may comprise amultiple-electrode electron reflector to which voltage levels arecontrollably applied. By setting the applied voltage levels, each pixelmay be set to one of two modes. In a first mode, the pixel may reflect afocused beamlet of electrons. In a second mode, the pixel may absorb orscatter electrons such that no focused beamlet of electrons is reflectedfrom that pixel. By setting only select pixels to reflect a focusedbeamlet, a patterned electron beam 113 may be generated by selectivereflection from the DPG 112.

The objective electron-optics 110 accelerates the patterned electronbeam 113 such that it passes the beam separator 106. The beam separator106 bends the trajectory of the patterned electron beam 113 towards theprojection electron-optics 114. The projection electron-optics 114 maycomprise an arrangement of magnetic and/or electrostatic lenses. Theprojection electron-optics 114 may be configured to focus and de-magnify(shrink) the patterned electron beam 113 such that it is projected ontophotoresist on a semiconductor wafer or onto other target substrate 118.

The stage 116 holds the target semiconductor wafer or other targetsubstrate 118. Depending on the implementation, the stage 116 may bestationary or in motion during the lithographic projection. In the casewhere the stage 116 is moving, the pattern on the DPG 112 may bedynamically adjusted to compensate for the motion such that theprojected pattern moves in correspondence with the wafer movement.

While FIG. 1 depicts an example reflection-mode system within which anembodiment of the invention may be implemented, embodiments of theinvention may be implemented within other reflection-mode systems aswell.

Further in regard to the multiple-electrode electron reflector which maybe used for pixels of the DPG 112, a cross-sectional diagram showing anexample reflector structure is provided in FIG. 2. As shown, thesidewalls surrounding the opening of each pixel well (cup) 202 comprisesa stack with multiple conductive layers or electrodes (for example, 211,212, 213, and 214) separated by insulating layers 210. In addition, eachwell includes a base electrode 220 at the bottom of each well 202.

The well 202 may be of a cylindrical shape such that the opening at thetop and the base electrode 220 at the bottom are circular. For example,each well 202 may have a diameter of 1.5 microns and may be 4 micronsdeep. The stacked electrode well structure may be fabricated on asilicon substrate 232 with an oxide layer 234 on the substrate. A CMOScircuit below the wells 202 may be used to apply the voltages to themultiple electrode layers.

Aberration Correction Apparatus for Reflection-Mode System

FIG. 3A is a planar view of an aberration correction apparatus 300 for areflection-mode system 100 in accordance with an embodiment of theinvention. More particularly, the planar view shows the aberrationcorrection apparatus 300 at the image plane at the DPG 112 of thereflection-mode system 100. As shown, in the image plane, the DPG 112 issurrounded by an inner electrode 302, and the inner electrode 302 is, inturn, surrounded by an outer electrode 304. A first voltage level Vinnermay be applied to the inner electrode 302, and a second voltage levelVouter may be applied to the outer electrode 304.

In the embodiment depicted in FIG. 3A, the outer perimeter of the innerelectrode 302 in the image plane may be circular and centered on theelectron-optical axis of the system. As further depicted, the outerperimeter of the outer electrode 304 in the image plane may be squareand centered on the electron-optical axis of the system. Other shapes ofthe inner electrode 302 and the outer electrode 304 in the image planeare also contemplated to be within the scope of the presently-disclosedinvention. For example, in another embodiment, the outer perimeter ofboth electrodes may be circular. In yet another embodiment, the outerperimeter of both electrodes may be square. In yet another embodiment,the outer perimeter of the inner electrode 302 may be square, and theouter perimeter of the outer electrode 304 may be circular.

FIG. 3B depicts a cross-sectional view of an aberration correctionapparatus 300 for a reflection-mode system in accordance with anembodiment of the invention. The cross section depicted is of the A-A′plane shown in FIG. 3A. The optical axis (OA) of the system is shown asgoing to a center of the DPG 112. As depicted, the inner and outerelectrodes (302 and 304) may be formed on an oxide layer 324 over asemiconductor substrate 322. The voltage levels Vinner and Vouter may beapplied via conductive conduits which go through the oxide layer 324 tothe inner and outer electrodes (302 and 304). In addition, there may bean insulating border (for example, of oxide) 305 between the DPG 112 andthe inner electrode 302 and an insulating border 306 between the innerand outer electrodes (302 and 304). Further shown in FIG. 3B are ashield electrode 308 above the DPG 112 and a bottom lens electrode 310of an electrostatic lens in the objective electron-optics 110. A shieldvoltage level, Vshield, may be applied to the shield electrode 308, anda lens voltage level, Vlens, may be applied to the bottom lens electrode310.

In accordance with an embodiment of the invention, the inner and outervoltage levels (Vinner and Vouter, respectively) are set so as tocorrect for curvature of field aberrations in the electron-optics. Inparticular, the Vinner and Vouter are set so as to modify the fielddistribution of the lens formed by the bottom lens electrode 310, theshield electrode 308, and the top electrode 211 of the DPG 112. Thefield distribution is modified so as to reduce aberrations of theoverall electron-optical system.

FIG. 4 is a planar view of an aberration correction apparatus 400 for areflection-mode system in accordance with another embodiment of theinvention. In this embodiment, instead of one inner electrode 302 andone outer electrode 304, there is one inner electrode 302 and four outerelectrodes (402, 404, 406 and 408). In this case, the four outerelectrodes (402, 404, 406 and 408) are arranged in four quadrants aroundthe inner electrode 302. In this embodiment, five separatelycontrollable voltages (Vinner, Vouter1, Vouter2, Vouter3 and Vouter4)are applied to the five electrodes (302, 402, 404, 406 and 408,respectively). The five voltages may be set so as to correct forcurvature of field aberrations and higher-order aberrations in theelectron-optics.

Transmission-Mode System

FIG. 5 is a simplified block diagram of a transmission-mode electronbeam lithography system 500 in accordance with an embodiment of theinvention. As shown, the system 500 may include an electron source 502,condenser electron-optics 504, a blanker array 506, projectionelectron-optics 508, and a stage 510 for holding a target substrate 512.

The condenser (or illumination) electron-optics 504 may be anarrangement of magnetic and/or electrostatic lenses which focuses andcollimates the electron beam from the electron source 502. In addition,the condenser electron-optics 504 allows the setting of the currentilluminating the blanker array 506.

The blanker array 506 may include a two-dimensional array of pixels.Various dimensions of the array may be implemented. Each pixel may beseparately controlled to either allow transmission of an electronbeamlet, or to block transmission of the electron beamlet (i.e. to“blank” the beamlet for the pixel). By setting only select pixels totransmit a beamlet, a patterned electron beam may be transmitted by theblanker array 506.

The projects electron-optics 508 may be an arrangement of magneticand/or electrostatic lenses which projects and de-magnifies (shrinks)the electron beam onto the surface of the target substrate 512. Thetarget substrate 512 may be held by a stage 510. Depending on theimplementation, the stage 510 may be stationary or in motion during thelithographic projection. In the case where the stage 510 is moving, thepattern on the blanker array 506 may be dynamically adjusted tocompensate for the motion such that the projected pattern moves incorrespondence with the movement of the target substrate.

Aberration Correction Apparatus for Transmission-Mode System

FIG. 6A is a planar view of an aberration correction apparatus for atransmission-mode system in accordance with an embodiment of theinvention. More particularly, the planar view shows the aberrationcorrection apparatus 600 at the pupil plane at the blanker array 506 ofthe reflection-mode system 500.

As shown, in the pupil plane, the blanker array 506 may be surrounded byan inner electrode 602, and athe inner electrode 302 may be, in turn,surrounded by an outer electrode 604. A first voltage level Vinner maybe applied to the inner electrode 602, and a second voltage level Voutermay be applied to the outer electrode 604.

In the embodiment depicted in FIG. 6A, the outer perimeter of the innerelectrode 602 in the pupil plane may be circular and centered on theelectron-optical axis of the system. As further depicted, the outerperimeter of the outer electrode 604 in the pupil plane may be squareand centered on the electron-optical axis of the system. Other shapes ofthe inner electrode 602 and the outer electrode 604 in the pupil planeare also contemplated to be within the scope of the presently-disclosedinvention. For example, in another embodiment, the outer perimeter ofboth electrodes may be circular. In yet another embodiment, the outerperimeter of both electrodes may be square. In yet another embodiment,the outer perimeter of the inner electrode 602 may be square, and theouter perimeter of the outer electrode 604 may be circular.

FIG. 6B is a cross-sectional view of an aberration correction apparatusfor a transmission-mode system in accordance with an embodiment of theinvention. The cross section depicted is of the A-A′ plane shown in FIG.6A. The optical axis (OA) of the system is shown as going through acenter of the blanker array 506. As depicted, there may be an insulatingborder (for example, of oxide) 605 between the blanker array 506 and theinner electrode 602 and an insulating border 606 between the inner andouter electrodes (602 and 604). In addition, the voltage levels Vinnerand Vouter may be applied, respectively, to the inner and outerelectrodes (602 and 604).

FIG. 7 is a planar view of an aberration correction apparatus for atransmission-mode system in accordance with another embodiment of theinvention. In this embodiment, instead of one inner electrode 602 andone outer electrode 604, there is one inner electrode 602 and four outerelectrodes (702, 704, 706 and 708). In this case, the four outerelectrodes (702, 704, 706 and 708) are arranged in four quadrants aroundthe inner electrode 602. In this embodiment, five separatelycontrollable voltages (Vinner, Vouter1, Vouter2, Vouter3 and Vouter4)are applied to the five electrodes (602, 702, 704, 706 and 708,respectively). The five voltages may be set so as to correct forcurvature of field aberrations and higher-order aberrations in theelectron-optics.

In another embodiment of the invention, the aberration correctionapparatus may be used more generally in electron beam based systems,including systems utilized for electron beam inspection, or defectreview, or metrology. For such an apparatus, the inner electrode maysurround either an opening through which an electron beam passes, or amirror electrode reflector which reflects the electron beam (instead of,a blanker array 506 or a DPG 112, respectively, in the above-describedlithography systems). The apparatus may be operated to reduce aberrationin the electron beam based system. An example of such an apparatus 800is shown in FIG. 8, where a circular-shaped opening or reflector 805 isshown within the inner electrode 802, and four outer electrodes 812,814, 816, and 818 are shown surrounding the inner electrode 802. Theplane shown in the figure would be a pupil plane if the inner electrodesurrounds an opening and would be an image plane if the inner electrodesurrounds a reflector.

The above-described diagrams are not necessarily to scale and areintended be illustrative and not limiting to a particularimplementation. In the above description, numerous specific details aregiven to provide a thorough understanding of embodiments of theinvention. However, the above description of illustrated embodiments ofthe invention is not intended to be exhaustive or to limit the inventionto the precise forms disclosed. One skilled in the relevant art willrecognize that the invention can be practiced without one or more of thespecific details, or with other methods, components, etc. In otherinstances, well-known structures or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An apparatus for aberration correction inelectron-optics of an electron beam system, the apparatus comprising: aninner electrode surrounding an opening in the electron-optics, the innerelectrode having a continuous annular planar surface extending unbrokenfrom a circular inner perimeter to a circular outer perimeter in a planeof the opening; at least one outer electrode around the inner electrode,the at least one outer electrode having a planar surface in the plane ofthe opening; circuitry configured to apply an inner voltage level to theinner electrode and at least one outer voltage level to the at least oneouter electrode; and a blanker array within the opening which receivesan incident electron beam and generates multiple beamlets of electrons,wherein the inner voltage level and the at least one outer voltagelevels, as applied to said electrodes, each causes an electric fieldthat influences both electrons of the incident electron beam that areincident to the opening and the multiple beamlets of electrons outgoingfrom the opening.
 2. The apparatus of claim 1, wherein each of the innervoltage level and the at least one outer voltage level are controlledindividually.
 3. The apparatus of claim 2, wherein the inner voltagelevel and the at least one outer voltage level are set to correct acurvature of field in the electron-optics.
 4. The apparatus of claim 1,wherein the inner electrode is centered on an optical axis of theelectron-optics.
 5. The apparatus of claim 1, wherein the at least oneouter electrode is a single outer electrode which is centered on anoptical axis.
 6. The apparatus of claim 1, wherein the at least oneouter electrode comprises a plurality of outer electrodes.
 7. Theapparatus of claim 1, wherein the at least one outer electrode comprisesfour electrodes which are arranged in quadrants around the innerelectrode.
 8. The apparatus of claim 7, wherein the inner voltage leveland the at least one outer voltage level are set to correct a curvatureof field and higher-order aberrations in the electron-optics.
 9. Theapparatus of claim 1, wherein the planar surfaces of the inner electrodeand the at least one outer electrode are in an image plane of theelectron-optics.
 10. The apparatus of claim 1, wherein the planarsurfaces of the inner electrode and the at least one outer electrode arein a pupil plane of the electron-optics.
 11. A method of correctingaberration in electron-optics of an electron beam system, the methodcomprising: adjusting an inner voltage level applied to an innerelectrode surrounding an opening, the inner electrode having acontinuous annular planar surface extending unbroken from an innercircumference to an outer circumference in a plane of the opening; andadjusting at least one outer voltage level which is applied to at leastone outer electrode around the inner electrode, the at least one outerelectrode having a planar surface in the plane of the opening, whereinthe inner voltage level and the at least one outer voltage levels, asapplied to said electrodes, each causes an electric field thatinfluences both a beam of electrons incident to the opening and multiplebeamlets of electrons outgoing from a blanker array in the opening. 12.The method of claim 11, wherein the inner electrode has an annularplanar surface with circular inner and outer perimeters.
 13. The methodof claim 11, wherein the inner voltage level and the at least one outervoltage level are set to correct a curvature of field in theelectron-optics.
 14. The method of claim 11, wherein the at least oneouter electrodes comprises a plurality of outer electrodes arrangedaround the inner electrode.
 15. The method of claim 14, wherein theplurality of outer electrodes comprises four electrodes which arearranged in quadrants around the inner electrode.
 16. The method ofclaim 15, wherein inner voltage level and the at least one outer voltagelevel are set to correct a curvature of field and higher-orderaberrations in the electron-optics.
 17. An electron beam columncomprising: an electron source for generating an electron beam thattravels along an optical axis of the electron beam column; condenserelectron-optics in the electron beam column for focusing and collimatingthe electron beam; an inner electrode in the electron beam column thatis positioned in a pupil plane of the electron beam column and has acontinuous annular shape surrounding an opening through which theelectron beam is transmitted and extending unbroken from an innercircumference to an outer circumference, wherein the optical axis isnormal to the pupil plane; at least one outer electrode positioned inthe pupil plane so as to surround the inner electrode; a two-dimensionalarray positioned in the pupil plane within the opening of the innerelectrode so as to receive the electron beam, wherein thetwo-dimensional array comprises a blanker array having pixels that areseparately controlled to allow or block transmission of electronbeamlets so as to form a patterned electron beam; and circuitryconfigured to apply an inner voltage level to the inner electrode and atleast one outer voltage level to the at least one outer electrode. 18.The electron beam column of claim 17, further comprising:projection-electron optics which projects and de-magnifies the electronbeam onto a surface of a target substrate.